The construction of propeller blades for aeroplanes and for the propulsion of air in general is costly when high performance is a requirement; with the involvement of laminated wood and glass fiber encasements or the use of complex castings and machined metal forgings therefor. The number of blades will vary from two to three or four and more, it being common practice to provide air circulation fans with a plurality of blades greatly in excess of the pair of or several blades which characterize the propellers of small aircraft, for example. Heretofore, resort has been made to the use of extrusions in the formation of propeller blade cross sections, in place of the characteristic screw shaped wooden or metallic blades that vary in cross section from root to tip. However, the uniform cross section which has characterized extruded propeller blades has not been conducive to ultimate efficiency, and to this end it is an object of this invention to provide the economy of extruded blading with the efficiency of sophisticated blade design having optimum pitch and cross sectional configuration varying from root to tip, as may be requred. In practice, the circular speed of the propeller blade cross section increases from root to tip, being subsonic toward the root and often supersonic toward the tip. Consequently, efficiency must be achieved by means of increased pitch toward the root, and conversely by means of decreased pitch toward the tip in order to preclude high speed stall, and further by means of progressively higher speed blade cross sections toward the tip. These advantages are attained with the present invention.
Sophisticated propeller blading combines the aforementioned features relating to aerodynamic requirements and structural requirements as well. Accordingly, propeller blades are tapered commensurate with the stresses to be encountered thereby, and consequently are of thicker cross section toward the root and of thinner cross section toward the tip. Heretofore, extruded propeller blading has not been conducive to a tapered configuration, however it is an object of this invention to taper extruded propeller blading for the combined purposes of advantageously varying the aerodynamics of its cross section from root to tip and of reducing its strength from root to tip. With the present invention, optimum aerodynamics and strength is realized.
The material substance of propeller blading and mode of manufacture is of great concern, since the laminate, cast and forged structures of the accepted prior art blading are very expensive and vulnerable to damage beyond repair; for instance, with the slightest damage an aircraft propeller is no longer airworthy, and to this end it is an object of this invention to provide a material substance and its formation which is the least vulnerable to damage and which inherently meets the requirements of airworthiness; and which in the event of damage is replaceable at a minimum of cost. With the present invention, an aluminum alloy extrusion is used in preference to a wood laminate, cast or forged aluminum member, at great savings in cost of material and working required to fabricate the same. As will be described, the mass of the propeller blade is minimized in the wrought cross section that varies in both shape and pitch from root to tip. As a result, there is root stiffness and tip flexibility, and all of which is replaceable on a mounting stem.
Heretofore, extruded propeller blading has been restricted to the straightened and uniform cross section of its basic form, and inherently inefficient for lack of sophistication, as pointed out above. Attempts have been made in the past to twist extruded blades, but unsuccessfully for lack of uniformity in the progressive variation of pitch related to cross section. It is an object therefore, of this invention to provide a method that is simple and practical by which a uniform and predictable variation in pitch from root to tip is successfully obtained, after a minimum amount of machining is performed so as to establish constant varying optimum cross sectional configurations. With the method hereinafter disclosed, the blade is twisted progressively by means of simultaneously applying twisting forces both above and below the elastic limit of the malleable material substance forming the blade.
The higher as compared with lower velocity airfoils vary greatly from root to tip of propeller blades, with respect to length of chord the front and back cambers, and the helical pitch. Further, fineness or depth of the airfoil diminishes progressively toward the tip; the front camber being convex throughout the blade length, and the back camber being concaved toward the root and flattened toward the tip. Therefore, it is an object to include these advantageous features in an extruded airfoil for propeller blading, by extruding a basic airfoil configuration that presents a useable front camber configuration, and a rear configuration that is partially removed so as to provide a flattened back camber that retains a convexely narrowing leading edge, a concaved trailing edge toward the root, and a convex trailing edge toward the tip. With the present invention, extrusion modification is by machining at the rear side thereof, truncation thereof to reduce chord length toward the tip being coordinated with back side reduction so as to control and establish thinness at the trailing edge. The removal of material substance at the back of the blade precedes the aforementioned progressive twisting into helical form.